Apertured probes for localized measurements of a material&#39;s complex permittivity and fabrication method

ABSTRACT

A probe for non-destructive determination of complex permittivity of a material and for near field optical microscopy is based on a balanced multi-conductor transmission line structure created on a dielectric substrate member which confines the probing field within a sharply defined sampling volume in the material under study. A method for manufacturing dielectric support member based probes includes anisotropically depositing a 50-100 Å thick underlayer of Cr, Ni, W or Ta onto the dielectric support member, anisotropically depositing conductive material onto the Cr, Ni, W or Ta underlayer, and removing the unwanted conductive material at the sides of the dielectric support member to electrically isolate the created conductive strips.

[0001] This Patent Application is a Continuation-in-Part of a patentapplication Ser. No. 09/665,370, filed on Sep. 20, 2000 “Apparatus forLocalized Measurement of Complex Permittivity of a Material”.

FIELD OF THE INVENTION

[0002] The present invention relates to measurement techniques. Inparticular this invention directs itself to a technique for highlylocalized measurements of complex microwave permittivity of materialsand for near-field optical microscopy.

[0003] More in particular, the present invention relates to a probe fornondestructive determination of complex permittivity of a material basedon a balanced multi-conductor transmission line resonator which providesconfinement of a probing field within a sharply defined sampling volumeof the material under study to yield a localized determination of thematerial's complex

[0004] Furthermore, the present invention relates to fabrication ofelongated dielectric support member based probes for localizedmeasurements of complex permittivity of materials at frequencies fromabout 10 MHz to 100 GHz, where the fabrication process involves coatingof an elongated dielectric support member with conducting material whichis then processed to remove the conducting material from predeterminedsides of the dielectric to yield a dielectric support member coated witha multi-conductor transmission line.

BACKGROUND OF THE INVENTION

[0005] One of the main goals of near-field scanning microwave microscopyis to quantitatively measure a material's complex microwave permittivity(dielectric constant and conductivity) with a high sensitivity oflateral and/or depth selectivity (i.e. to determine the material'sproperty over a small volume while ignoring the contribution of thatvolume's surrounding environment). This is particularly important inmeasurements on complex structures, such as semiconductor devices orcomposite materials, where, for example, the permittivity of one line orlayer must be determined without having knowledge of the properties ofthe neighboring lines or underlying layers.

[0006] In microwave microscopy a basic measurement is a determination ofthe reflection of a microwave signal from a probe positioned in closeproximity to a sample. Phase and amplitude of the reflected signal maybe determined directly by using a vector network analyzer or bydetermination of the resonant frequency and quality factor of aresonator coupled to the probe.

[0007] In many cases, the phase of the reflected signal correlates to alarge extent with the real part of the sample permittivity, whereasmagnitude is dominated by the imaginary part of the permittivity (i.e.,the microwave absorption of the sample). Measurements of the microwavetransmission from the probe through the sample are also possible,however, such an arrangement generally does not yield a localizeddetermination of a sample's complex permittivity.

[0008] The most typical approaches in microwave microscopy employ acoaxial probe geometry also referred to as apertureless probes, in whicha central inner conductor (usually an STM tip) protrudes from one end ofthe probe and is tapered, as shown in FIG. 1. An alternative to therotationally-symmetric arrangement of the coaxial probes are planarstructures such as a co-planar wave-guide or a strip-line wave-guide.The tapered tip is used for concentrating the electric field aroundand/or underneath the tip apex which permits the probes to ‘image’features on the order of the tip apex curvature or less. This ‘imaging’resolution however, is not a quantitative measurement since the probe isaveraging over a volume that is usually a few orders of magnitude larger(usually a few millimeters) than the tip apex curvature. While the fieldconcentration around the tip apex is significant, there are also fieldsthat extend over much larger distances. Such an apparatus yields animaging resolution on the order of the diameter or radius of curvatureof the central conductor tip.

[0009] It is obvious from considerations of classic electrodynamics thatthe volume of space over which an apparatus determines the electricalproperties of a sample is determined not by the local dimensions of thecentral conductor tip alone, but rather by a length scale given by theseparation between the central conductor tip and the ground (outer)conductor or shield, as shown in FIG. 1.

[0010] Therefore, in order to quantitatively determine the microwaveproperties of a material these properties must be devoid ofnon-uniformities on length scales of at least several times larger thanthe distance between the probe tip and the ground conductor whilesufficient imaging contrast on length scales comparable to the radius ofcurvature of the tip may be achieved.

[0011] It is further obvious from considerations of classicalelectrodynamics that the separation between the probe and a sampleaffects the capacitance measured which is a function of the probe-sampleseparation and the electrical field distribution. Thus, it is importantthat the separation between the probe and the sample be measured inorder to determine complex permittivity in a non-contact manner. Withoutaccurate control of distance and a small volume of electrical fielddistribution high lateral and/or depth selectivity and accuratequantitative results cannot be achieved with conventional technology.

[0012] Furthermore, the inherent unbalanced character of the exposedportion of the probe complicates the above-mentioned geometries due tothe dipole-like current-flow in the surrounding area. The amount ofradiation is critically dependent on the environment, i.e., the sample'scomplex permittivity and the probe-to-sample distance which affects theamplitude of the reflected signal (reflection measurement) or qualityfactor of the resonator (resonant technique). The result is apotentially erroneous determination of the sample's microwaveabsorption.

[0013] Conventional near-field microwave probes cannot be used forsimultaneous near-field optical measurements and complex permittivitymeasurements due to the fact that the tapered fiber tip serves as acircular waveguide disadvantageously having a cut-off frequency for theoptical and microwave radiation.

SUMMARY OF THE INVENTION

[0014] An object of the present invention is to provide a novel probefor the nondestructive determination of a sample's complex permittivitybased on a balanced multi-conductor transmission line resonator which issymmetric with respect to an exchange of signal between the conductors.This permits confinement of the probing field within the desiredsampling volume to significantly reduce dependency of measurements onthe sample volume's environment.

[0015] It is another object of the present invention to provide a methodfor fabrication of a dielectric support member based (including afiber-optic based) near-field microwave probe by means of coating anelongated dielectric support member with a conducting material which isfurther processed in subsequent technological steps to be removed from apredetermined number of sides of the dielectric support member resultingin the dielectric support member coated with a multi-conductortransmission line which can be used as a probe for complex permittivitymeasurements as well as providing a tip for near field opticalmicroscopy.

[0016] In accordance with the principles herein presented, the presentinvention provides a novel probe for non-destructive measurements whichincludes a multi-conductor (preferably, a two-conductor) transmissionline created on the dielectric support member. One end of thetransmission line (also referred to herein as the “probing end”) isbrought into close proximity to the sample to be measured and may betapered (or sharpened) to an end having a minimal spatial extent. Asignal is fed through the transmission line toward the sample and asignal reflected from the sample is measured. The opposite end of thetransmission line is connected to electronics for the determination ofthe reflected signal's phase and magnitude. Measurements of the phaseand magnitude of the reflected signal are broadband in frequency.Alternatively, if the probe is coupled to a resonator, the electronicsthen determine the resonant frequency and quality factor of theresonator which results in a measurement which is narrowband infrequency.

[0017] In this type of system, the diameter of the tip portion of theprobe is in the range from 50 nm to 10 μm with the diameter of theelongated dielectric support member being in the range from 10 μm to 10mm. The conductive strips are separated one from the other around thedielectric support member by a distance not to exceed approximately 100nm.

[0018] Each conductive strip is formed from a conductive material from agroup which includes: metallic or superconducting materials, Au, Ag, Cu,Al, YBCO, Cr, W, Pt, Nb, etc., and mixtures thereof Each conductivestrip is formed upon a Cr, Ni, W, or Ta underlayer of 50-100 Å thicknessdirectly deposited on the dielectric support member (or onto a claddinglayer if the dielectric support member is fiber-optic based structure).

[0019] In accordance with the subject invention concept, such adielectric support may be in the form of any of the followingembodiments:

[0020] a) Optical fiber with exposed cladding, having bare (untapered)cladding diameter in the range of 10 μm to 10 mm. The fiber may beformed from any insulating material that can be tapered by means ofetching and/or heating/pulling, and has a dielectric loss tangent ≦10⁻¹at the operating frequency (e.g. quartz, sapphire, glass, etc.)

[0021] b) Dielectric rod (substantially circular in cross-section) from10 μm to 10 mm in outer diameter formed from any insulating materialthat may be tapered by means of etching and/or heating/pulling and has adielectric loss tangent ≦10 ⁻¹ at the operating frequency (e.g. quartz,sapphire, glass, etc.)

[0022] c) Dielectric tube (micropipette, etc.) from 10 μm to 10 mm inouter diameter and appropriate inner diameter formed from any insulatingmaterial that can be tapered by means of etching and/or heating/pullingand has a dielectric loss tangent ≦10⁻¹ at the operating frequency (e.g.quartz, sapphire, glass, etc.)

[0023] d) Dielectric tube (micropipette, etc.) from 10 μm to 10 mm inouter diameter and appropriate inner diameter formed from any insulatingmaterial that may be tapered by means of etching and/or heating/pulling,and has a dielectric loss tangent ≦10⁻¹ at the operating frequency (e.g.quartz, sapphire, glass, etc.) with an optical fiber inserted into thetube.

[0024] e) Dielectric bar of square, rectangular, pentagonal, hexagonal,octagonal, etc. cross-section with the cross-section linear dimensionsfrom 10 μm to 10 mm formed from any insulating material that may betapered by means of etching and or heating/pulling and has a dielectricloss tangent ≦10⁻¹ at the operating frequency (e.g. quartz, sapphire,glass, etc.)

[0025] f) multi-barrel dielectric tubing (with the number of barrelsfrom 2 to 20) or Theta-tube formed from anv insulator that may betapered by means of etching and/or heating/pulling and which has adielectric loss tangent ≦10⁻¹ at the operating frequency (e.g. quartz,sapphire, glass, etc.). One or more of the barrels may have an insertedoptical fiber or metal wire.

[0026] The present invention is further directed to a method formanufacturing dielectric support member based probes, includingfiber-based probes for near-field optical microscopy and/or complexpermittivity measurements. For manufacturing the fiber-based probes, themethod includes a preliminary step of:

[0027] removing an outer jacket from a fiber-optic wire of apredetermined length to expose a cladding layer surrounding a centraloptical fiber of the fiber-optic wire and

[0028] anisotropically depositing a 50-100 Å thick underlayer of Cr, Ni,W or Ta onto the cladding layer. For all other dielectric support memberbased probes, a 50-100 Å thick under layer of Cr, Ni, W or Ta isanisotropically deposited directly onto the dielectric support member.

[0029] The method further includes:

[0030] optionally removing the Cr, Ni, W or Ta underlayer betweenpredetermined locations;

[0031] anisotropically depositing a conductive material on the Cr, Ni, Wor Ta underlayer at predetermined locations; and

[0032] removing the conductive material between the predeterminedlocations, to form a plurality of electrically isolated conductivestrips on the elongated dielectric support member (including fiber opticwire).

[0033] The conductive material and the Cr, Ni, W or Ta underlayer aredeposited by one of any known conventional deposition techniques, forexample, Pulsed Energy Deposition, Evaporation, Sputtering, Dipping,Focused Ion Beam Deposition, etc.

[0034] The conductive material and the Cr, Ni, W or Ta underlayer areremoved by means of l number of material removal techniques, such as:Ion Beam Milling, Focused Ion Beam Milling, Chemical Etching, Mask-LessPhoto-Lithography, etc.

[0035] The removal of the Cr, Ni, W or Ta underlayer may be omittedsince it has been found that the method works with or without this stepbeing incorporated.

[0036] For all of the aforesaid embodiments of the probe 10 of thepresent invention the procedure for fabrication of the probes issubstantially the same and includes the following steps:

[0037] (i) The elongated dielectric support member is tapered down bymeans of chemical etchina (e.g. using HF, etc.) and/or heating/pulling(e.g. using CO₂ laser or heating filament based puller) thus forming atip at the end with the apex curvature (or diameter) down to 10 nm;

[0038] (ii) The tapered dielectric support member is coated with aconducting material (e.g. Cu, Al, Ag, Au, etc.). Multi-layer coating maybe used (e.g. Cr/Au) in this step.

[0039] (iii) The conducting material is removed from two or more sidesto produce a multi-conductor transmission line that may be used as aprobe for complex permittivity measurements.

[0040] For the embodiment (f), the procedure for manufacturing of theprobe may include the following procedures:

[0041] (i) Two (or more) metallic wires (e.g. Cu, Al, Ag, Au, etc.) ofappropriate diameter are inserted into the two (or more) differentopenings inside the multi-barrel or the capillary Theta-tube made ofquartz;

[0042] (ii) The assembly obtained in (i) is tapered by means ofheating/pulling (e.g. using CO, laser or heating filament based puller)thus forming a tip with the apex curvature (or diameter) down to 10 nm;

[0043] (iii) Optionally a tapered structure is uniformly coated with aconducting material (e.g. Cu, Al, Ag, Au, etc.) forming a shieldedbalanced transmission line. Additionally, a multi-layer coating may beused (e.g. Cr/Au). Such a structure permits construction of a balancedmicrowave transmission line and/or a resonator due to generally lowmicrowave losses in quartz.

[0044] These and other novel features and advantages of this inventionwill be fully understood from the following detailed description of theaccompanying Drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045]FIG. 1 is a schematic representation of an apertureless probe of aprior art system;

[0046]FIG. 2 schematically depicts a resonator formed by a dielectricsupport member based probe of the present invention having a taperedwire with a pair of metallic strips deposited thereon;

[0047]FIG. 3 schematically describes a probe of the present inventionformed by an untapered dielectric support member with two conductivestrips deposited thereon;

[0048] FIGS. 4A-4G is a schematic representation of a technologicalsequence of fabrication of the dielectric support member based probe ofthe present invention;

[0049] FIGS. 5A-5D is a schematic representation of the mask-lessphotolithography technique of the present invention;

[0050] FIGS. 6A-6B represent schematically the technique formanufacturing the probes in the form of multi-barrel dielectric tubing;

[0051] FIGS. 7A-7B is a schematic representation of the focused ion beam(FIB) cut technique for making the tip of the dielectric support memberbased near-field probe;

[0052] FIGS. 8A-8C is a schematic representation of the ion millingtechnique for fabricating the tip of the probe of the present invention;

[0053] FIGS. 9A-9C is a schematic representation of the FIB erasingtechnique for fabricating the tip of the probe of the present invention;and

[0054] FIGS. 10A-10B is a schematic representation of the FIB techniquefor drawing, strip lines onto the tip of the probe of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0055] Referring to FIGS. 2-3, there is shown a probe 10 fornon-destructive determination of the complex permittivity of a sample11. The probe 10 is designed as a dielectric support member based twoconductor transmission line 12. The support member for the probe 10 isformed of dielectric (i.e., insulating) material that is elongated alongone axis, and may have varying dimensions of its cross-section along thelength thereof. The probe 10 includes two (or more) spatially separatedsymmetrically arranged electrical conductors (conductive strips) 13 and14 deposited on a surface thereof or extending internally along thelength of the dielectric support.

[0056] The dielectric support member may be in the form of any of thefollowing embodiments:

[0057] a) Optical fiber with exposed cladding, with bare (untapered)cladding diameter in the range of 10 μm to 10 mm. The fiber may beformed from an insulating material that can be tapered by means ofetching and/or heating/pulling and has a dielectric loss tangent ≦10⁻¹at the operating frequency (e.g. quartz, sapphire, glass, etc.)

[0058] b) Dielectric rod (circular in cross-section thereof) fromapproximately 10 μm to 10 mm in outer diameter formed from an insulatingmaterial that can be tapered by means of etching and/or heating/pullingand has a dielectric loss tangent ≦10⁻¹ at the operating frequency (e.g.quartz, sapphire, glass, etc.)

[0059] c) Dielectric tube (micropipette, etc.) from 10 μm to 10 mm inouter diameter and appropriate inner diameter formed from an insulatingmaterial that can be tapered by means of etching and/or heating/pulling,and has a dielectric loss tangent −10⁻¹ at the operating frequency (e.g.quartz, sapphire, glass, etc.)

[0060] d) Dielectric tube (micropipette, etc.) from 10 μm to 10 mm inouter diameter and appropriate inner diameter formed from an insulatingmaterial that can be tapered by means of etching and/or heating/pulling,and has a dielectric loss tangent ≦10⁻¹ at the operating frequency (e.g.quartz, sapphire, glass, etc.) with an optical fiber inserted into thetube.

[0061] e) Dielectric bar of square, rectangular, pentagonal, hexagonal,octagonal, etc. cross-section with the cross-section linear dimensionsfrom 10 μm to 10 mm formed from an insulating material that can betapered by means of etching and/or heating/pulling and has a dielectricloss tangent ≦10⁻¹ at the operating frequency (e.g. quartz sapphire,glass, etc.)

[0062] (f) Multi-barrel dielectric tubing (with the number of barrelsfrom 2 to 20) or Theta-tube formed from an insulator that can be taperedby means of etching and/or heating/pulling and which has a dielectricloss tangent ≦10⁻¹ at the operating frequency (e.g. quartz, sapphire,glass, etc.) With regard to the multi-barrel dielectric tubing, one ormore of the barrels may have an inserted optical fiber or metal wire.

[0063] With regard to the fiber-optic-based embodiment of the probe 10of the present invention specifically shown in FIGS. 2-3, the two (ormore) spatially separated symmetrically arranged electrical conductors(also referred to herein as conductive elements) 13 and 14 are depositedon a surface of the cladding layer 15 of a fiber-optic wire 16. For allother embodiments referenced above, the conductive strips (or conductiveelements) 13 and 14 are formed directly on the surface of the dielectricsupport member.

[0064] For the aforesaid embodiments of the probe 10 of the presentinvention, the procedure for manufacturing of the probes issubstantially the same; and includes the following steps:

[0065] (i) The dielectric support member is tapered down by means ofchemical etching (e.g. using HF, etc.) and/or heating/pulling (e.g.using CO₂ laser or heating filament based puller) thus forming a tip atthe end with the apex curvature (or diameter) down to 10 nm;

[0066] (ii) The tapered dielectric support member is coated with aconducting material (e.g. Cu, Al, Ag, Au, etc.). Multi-layer coatingscan be used (e.g. Cr/Au) in the overall coating step.

[0067] (iii) The conducting material is then removed from two or moresides to produce a multi-conductor transmission line which is used as aprobe for complex permittivity measurements.

[0068] For the embodiment (f), the procedure for manufacturing of theprobe may include the following procedures:

[0069] (i) Two (or more) metallic wires (e.g. Cu, Al, Ag, Au, etc.) ofappropriate diameter are inserted into the two (or more) differentopenings inside to multi-barrel or the capillary theta-tube formed ofquartz;

[0070] (ii) The assembly obtained in i) is tapered by means of heatingpulling (e.g. using CO₂ laser or heating filament based puller) thusforming a tip with the apex curvature (or diameter) having a dimensionas low as 10 nm:

[0071] (iii) Optionally, such a tapered structure is uniformly coatedwith a conducting material (e.g. Cu, Al, Ag, Au, etc.), thus forming ashielded balanced transmission line. As previously noted, multi-layercoatings can be used (e.g. Cr/Au). Such a structure permits constructionof a balanced microwave transmission line and/or a resonator due to lowmicrowave losses in quartz.

[0072] As shown in FIGS. 6A-6B, the probe 10 is made from a Theta tube(or multi-barrel dielectric tubing) 60 having two or more channels 62(the tube 60 is made of a material having a dielectric loss tangent<10⁻³ at the operating frequency).

[0073] In each of the channels 62, a metal wire (or a fiber-optic wire)64 with a thickness of about few hundred microns is inserted. Theassembly thus formed is tapered down initially to approximately 10 nm bymeans of etching and/or heating/pulling (using for example CO₂ lasers orheating filament based puller) thus forming a probe 10 having a tip 66with the diameter 67 down to 10 nm, a diameter at the end 70 thereofapproximately 1 mm, and the length 72 corresponding to λ/4 or λ/2resonator.

[0074] The sheath 68 is further deposited onto the quartz tube 60 asshown in FIG. 6B, leaving the tip 66 free of the sheath. However, thesheath may be optionally extended down to the apex, as shown ininterrupted lines in FIG. 6A, in order that the measurements withapproximately the same spatial resolution can be made with this probe inboth modes, such as co-axial and two-conductor probe geometries. Thesheath 68 is preferably formed of Cu, Al, Ag, Au, or alternatively amultilayer coating can be used, for example, containing Cr/Au.

[0075] When used for manufacturing a resonator, the Q factor of suchresonator with the probe shown in FIGS. 6A-6B may be a few hundred.

[0076] There are several techniques that have been developed forfabricating the tip for the dielectric support member based near fieldprobe. For example, as shown in FIG. 7A, the focused ion beam (FIB) 82is directed to the apex 84 of the tip of the tapered dielectric supportmember 21 covered by a metal 85 which may be Au. Such a techniqueproduces a clean cut exposing a dielectric support member 21 surroundedby a plurality of electrically isolated conductive strips as shown inFIG. 7B illustrating a cross-section of the tip of the probe. Asunderstood, the tip may be circular, rectangular, pentagonal, hexagonal,octagonal. etc., or otherwise contoured.

[0077] As shown in FIGS. 8A-8C, ion milling can be used for fabricatingthe tip of a near field probe. As shown in FIG. 8A, an ion beam 86 isdirected to the apex 84 of the probe, i.e., the dielectric supportmember 21 covered with the conductive material 88, and the probe isrotated in the direction shown by the arrow 90. Also, as shown in FIG.8B, the dielectric wire 21 may be bounced around the tip point as shownby the arrow 92 in order to permit the ion beam 86 to etch the gap 94with the width on the order of the tip curvature, as shown in FIG. 8C.This technique is relatively inexpensive, simple, and can be donein-situ.

[0078] Shown in FIGS. 9A-9C is another technique of fabricating the tipsof the probes of the present invention, i.e., FIB erasing. According tothis technique, the Au (Ag, Al, etc.) is deposited onto the dielectricsupport member 21 and ion milling is performed which is applied to theapex of the probe, similar to the procedure shown in FIGS. 8A and 8B, orusing FIB 82 to make a channel in the tip. The diameter of the channelmay be approximately 5 nm. The tip is then cut off with FIB 82 as shownin FIG. 9B, or the FIB 82 can be scanned around the tip, as shown inFIG. 9C.

[0079] As shown in FIGS. 10A and 10B, the FIB may also be used to drawstrip lines on the dielectric support member. In this manner, thefocused ion beam 96 is directed towards the tip of the probe (dielectricmaterial of the dielectric wire 21 is exposed from the metal cover 88),and is reciprocally moved in the direction shown by the arrow 98 so thatparticulates of a conductive material are deposited onto the tip of theprobe to form a narrow line 100 with the thickness of the line beingapproximately 5 nm.

[0080] The embodiments (a), (d), and (f) of the probe of the presentinvention that include an optical fiber in addition to complexpermittivity measurements may also be used for performing simultaneousnear-field optical microscopy measurements. Additionally, embodiments(c) and (f) may be used to measure the permittivity of very smallamounts of liquid by means of pulling the liquid into the capillarytube.

[0081] Referring again to FIGS. 2 and 3, to conduct measurements, aprobing end 17 of the transmission line 12 is brought in close proximityto the sample 11, while an opposite end 18 of the transmission line 12is either connected to electronics for the determination of a reflectedsignal's phase and magnitude, or to a terminating plate 19 to form aresonator structure 20 for purposes described in following paragraphs.

[0082] The probe 10 is primarily envisioned in two functionalembodiments:

[0083] A. In operation as a transmission line for feeding a signal tothe sample 11 and measuring the phase as well as the magnitude of thereflected signal. This transmission line is operated either in the oddmode, i.e., in a mode in which the current flow in one of the twoconductors for example, the conductor 13, is opposite in direction tothat in the other conductor 14; or, in an even mode where a conductingsheath is used for enveloping the transmission line 12. When operated inthe even mode, the interaction between the sample and the probe issimilar to the coaxial symmetries known to those skilled in the art.Measurements of the phase and magnitude of the reflected signal by meansof the transmission line arrangement are broadband in frequency but aregenerally not satisfactory with respect to sensitivity to the sampleproperties and require additional rather expensive and complexelectronic equipment to be used such As a vector network analyzer.

[0084] B. In order to obtain a more sensitive and accurate result whileemploying less expensive equipment, the probe 10 of the presentinvention is envisioned as a resonator structure 20 which is formed by aportion of the transmission line 12 with the conductors 13, 14 separatedby a dielectric medium of the dielectric support member 21 shown in FIG.2.

[0085] The probing end 17 of the resonator structure 20 is brought intoproximity to the sample 11 (which may be an ion-implanted silicon, metaland/or dielectric complex structure, metal and/or dielectric films onany substrate, etc.) with the opposite end 18 of the transmission lineresonator structure 20 being coupled to the terminating plate 19. Theresonator structure 20 is formed in order to measure the resonantfrequency and quality factor of the resonator structure 20 fordetermination of the complex permittivity of the sample 11.

[0086] The spacing between the conductors 13, 14, and theircross-section have to be properly chosen in order to maintain aresonator quality factor Q high enough for accurate measurements of thesample induced changes in the resonant frequency and the Q factor. Thedistance between the conductors 13 and 14 has to be on the order of orgreater than approximately 1 mm for Q>1000 at 10 GHz. This distance isdetermined by the diameter of the dielectric support member which may bein the range from 10 μm to 10 mm.

[0087] When the probe 10 of the present invention is operated as theresonator, the odd and even modes of operation in general result in twodifferent resonant frequencies due to dispersion of the signal and maytherefore be separated in the frequency domain and independently poweredand monitored. The dielectric medium of the support member 21 betweenthe conductors 13 and 14 serves to enhance such dispersion.

[0088] The coupling to the resonator 20 is accomplished by a couplingloop 22 positioned close to the resonator 20 inside an optionalconducting sheath 23 (as best shown in FIG. 2). An optional secondcoupling loop 24 may be used for the measurement electronics 25,schematically shown in FIG. 2. Alternatively, a circulator ordirectional coupler may be used to separate the signal reflected fromthe resonator 20 back into the feed loop.

[0089] The resonant frequency and quality factor of the resonatorstructure 20 may be determined by techniques known to those skilled inthe art. One commonly used configuration is shown in D. E. Steinhauer,C. P. Vlahacos, S. K. Dutta, F. C. Wellstood, and S. M. Anlage, AppliedPhysics Letters, Volume 71, Number 12, Sep. 22, 1997, pages 1736-1738.

[0090] In particular, a frequency-modulated microwave signal (typicallyat 5-10 GHz, 1 mW) is generated by a microwave source, such as the modelHP83752A manufactured by Agilent Technologies (Palo Alto, Calif.), whichis fed to the resonator. The reflected signal is routed via acirculator, such as model DMC6018 from DiTom (San Jose, Calif.) to adetector diode, such as model HP8473C from Agilent Technologies (PaloAlto, Calif.). The output of the diode is a voltage signal having acomponent at a frequency identical to that of the frequency modulationof the microwave source which can be accurately detected using a lock-inamplifier, such as model 7280 from Ametek, Inc. (Oak Ridge, Tenn.).

[0091] The voltage measured using the lock-in amplifier is proportionalto the difference between the resonance frequency and the carrierfrequency of the microwave source. A voltage component at twice thefrequency of the modulation of the microwave signal is proportional tothe quality factor of the resonator. This may again be measured using alock-in amplifier, such as model 7280 from Ametec, Inc. (Oak Ridge,Tenn.). The complex permittivity of the sample may be determined bycomparison of the measured quantities to calibration data obtained forknown materials.

[0092] The resonator structure 20 of the present invention forms eithera (2n+1)λ/4 or a (n+1) λ/2 resonator (n=0, 1, 2, . . . ), and its lengthis determined by the frequency of the lowest mode, e.g., about 7.5 mmfor the λ/4 mode at 10 GHz.

[0093] The resonator structure 20 may be enclosed in a cylindricalsheath 23 as shown in FIG. 2, for example, metallic or superconductingmaterials, Cr, W, Pt, Nb, Au, Ag, Al, YBCO and mixtures thereof. Thesheath 23 eliminates both radiation from the resonator 20 and the effectof environment interference on the resonator characteristics. Inparticular, the changing influence of moving parts in the proximity ofthe resonator 20 is eliminated. The sheath 23 has an opening 26 near thesample area. This permits an efficient coupling of the sample 11 to theresonator 20 and further permits the resonant frequency and Q factor tobe dependent on the sample microwave permittivity. Where the spacingbetween the conductors 13 and 14 is small in comparison to the innerdiameter of the sheath 23, the resonator properties are substantiallyunaffected by the sheath presence. The upper part of the sheath 23 makeselectrical contact with the terminating plate 19. The bottom portion ofthe sheath 23 may have a conical shape in order to provide clearphysical and visual access to the sampling area.

[0094] As discussed in previous paragraphs, the probing end 17 of theresonator structure 20 is brought into close proximity to the sample 11for measurement purposes. The geometry of the cross-section at theprobing end 17 determines the sampling volumes i.e. the spatialresolution both laterally and in depth. Due to the symmetry of thenear-field electrical field distribution at the probing end 17, theprobe 10 of the present invention permits a determination of thein-plane anisotropy of the complex permittivity of the sample 11. Inparticular, measurements (or entire scans) obtained with different probeorientations with respect to the sample 11 may be compared or subtractedeach from the other for the anisotropy determination.

[0095] If the sheath 23 is used with the probe 10, the probing end 17protrudes through and beyond the opening 26 formed in the sheath 23. Dueto the fact that there is weak coupling between the sheath 23 and theresonator 20, the diameter of the opening 26 affects neither the fieldintensity at the probing end 17, nor the Q factor of the resonator 20and further does not affect the sample's contribution to the resonator20 behavior.

[0096] However, for optimum spatial selectivity (quantitativeresolution), the diameter of the opening 26 is generally less than thelength of the proximal portion 27 of the resonator 20 that extendsbeyond the sheath 23. This eliminates the interaction between the sample11 and the weak near-field that is present in the immediate environmentof the opening 26. Due to the quadrupole-like current distribution inthe portion 27 of the resonator 20 outside the sheath this portion ofthe resonator 20 as well as the opening area 26 produces a negligibleamount of microwave far-field radiation. Additionally, substantially nomicrowave current is present on the exterior surface of the sheath 23,which has been confirmed by 3D numerical modeling. Hence, the probe 10produces significantly less radiation than conventional coaxialgeometries and mainly interacts with the sample 11 via the near-fieldcontribution.

[0097] In order to obtain high spatial resolution, in other words, inorder to reduce the size of the volume over which the microwaveproperties of a sample are determined, the diameter of the conductors 13and 14 at the probing end 17, as well as their spacing 21 must bereduced in size to the least possible dimension which may beaccomplished by tapering each of the two conductors 13 and 14 down tothe desired cross-section while simultaneously gradually reducing theirseparation as shown in FIG. 2.

[0098] Although the entire transmission line resonator 20 may be formedfrom a single piece of a dielectric material elongated along one axisand forming a cylindrical dielectric support member (including opticalfiber) with either a non-tapered or tapered probing end, as shown inFIG. 2 and described in previous paragraphs. It is also envisioned to bewithin the scope of the present invention that a portion of thetransmission line closest to the sample 11 may be replaced with adielectric support member 21 onto which two metallic strips 28 and 29are deposited on opposite sides of the dielectric support member 21 asshown in FIG. 3.

[0099] The two metallic strips 28 and 29 may be deposited on thedielectric support member in a manner to form either parallel untaperedmetallic lines, or conductive strips tapered to a sharp point andsimultaneously are gradually brought into close proximity to each other(as shown in FIG. 2). In the arrangement shown in FIG. 3, a clamp 30supports the dielectric support member in a predetermined orientationwith respect to the sample 11.

[0100] In the embodiments shown in FIGS. 2 and 3, the dielectric supportmember with two conducting metal strips is loaded into the resonator 20either via the through holes formed in the terminating plate 19 at thetop of the probe 10, or through the opening 26 in the sheath 23.

[0101] The distance between the probe 10 and the sample 11 is controlledby a tracking/control unit 31, schematically shown in FIG. 2. Thetracking/control unit 31 may include one of a number of distancedetection mechanisms known in the field of near field scanning opticalmicroscopy.

[0102] One of the embodiments of the probe 10 of the present invention,specifically the optical fiber based probe is fabricated on the base ofa standard optical fiber 16 which includes a central optical fiber 21surrounded by the cladding 15 and further surrounded by an outer jacket32, shown in FIG. 4A. The outer jacket 32 is stripped from the cladding15 to expose the cladding 15, as shown in FIG. 4B. For non-fiber basedembodiments of the probe 10 of the present invention the proceduresshown in FIGS. 4A and 4B are omitted. All further steps shown in FIGS.4C-4G for the optical fiber-based and non-fiber based probes aresimilar. In this regard, the dielectric support member may be furtheroptionally tapered as shown in FIG. 4C using standard techniques suchas:

[0103] 1. A heat-pull technique in accordance with which a laser beam isfocused on the dielectric elongated member while force is applied atboth ends of the dielectric member to stretch it. As the dielectricmember is stretched, it becomes smaller in diameter near the center,while generally maintaining its original diameter near the ends to whichthe stretching force is applied. Finally, the tapered wire is separatedinto two wires with tapered tips. For the purposes of the presentinvention these tips are formed with diameters down to 10 μm. The lasermay also be replaced with a coiled heating element, with the sameeffect.

[0104] 2. Chemical etching technique, in accordance with which an end ofthe dielectric support member is exposed to an etchant in a timedpredetermined manner, for example, by immersing the to-be tapered end ofthe dielectric support member into the etchant solution and pulling thedielectric support member from the solution at a predetermined speedthus creating a tapered profile of the dielectric member end.

[0105] Once the dielectric support member of the desired probe geometryand length is obtained, it is placed in the deposition chamber 33schematically shown in FIG. 4D, in order to anisotropically deposit Cr,Ni, W or Ta underlayer 34 at predetermined positions 35 and 36 along thedielectric member (or along the cladding 15 in the fiber-basedembodiment). The underlayer 34 may be deposited by Pulsed LaserDeposition, Evaporation, Dipping, Sputtering, or Focused Ion BeamDeposition. In order to perform the deposition/evaporation technique, asource 37 of Cr, Ni, W or Ta is positioned a predetermined distance fromthe dielectric support member within the deposition chamber 33 and theparticulates of Cr, Ni, W or Ta are directed from the source 37 to theside of the dielectric support member proximal to the source 37.

[0106] The source 37 of Cr, Ni, W or Ta may be either an evaporator boatfor performing thermal evaporation, or a deposition plume which may beused in accordance with known techniques of pulse laser deposition.Additionally, the deposition of material directly onto the surface ofthe dielectric support member or onto the surface of the cladding 15 maybe performed by conventional dipping techniques known to those skilledin the art. The deposition/evaporation technique provides for adirectional deposit of Cr (or other) material onto the side of thedielectric member facing the source of material.

[0107] After the source facing side is coated, the dielectric supportmember is rotated according to the number of conductors desired to bedeposited. As an example, a two conductor probe requires two depositionswith 180° rotation. A three conductor probe will require threedepositions with 120° rotation between them (the rotation angle is 360°divided by the number of conductors required). During deposition of Cr,Ni, W or Ta, sides 38 and 39 between the predetermined positions 35 and36 (where the conductive strips are to be deposited) also receives somematerial, the thickness of which is significantly less than at thelocations 35 and 36.

[0108] The result is that the thick layers 40 and 41 of Cr, Ni, W or Tawhich are positioned at the predetermined locations 35 and 36 may beconnected by a thin layer 42 and 43 of Cr, Ni, W or Ta present on thetwo “side walls” 38 and 39. These two thin layers 4′ and 43 may beoptionally removed in a following step, shown in FIG. 41E through ionbeam milling, focused ion beam milling, or by chemical etching. However,the Cr, Ni, W or Ta underlayer may be left unremoved since it has beenfound that the presence of the Cr, Ni, W or Ta underlayer does notdeteriorate the performance of the probe.

[0109] Etching solvents may also be used to remove conducting materialfrom a specific area on a surface. In this case a photoresist may beapplied to a sample and then selectively exposed to allow etchingsolvents to remove material only in the exposed area.

[0110] In one scenario, in order to remove the thin layers 42 and 43 ofCr, Ni, W or Ta and separate the two thick layers 40 and 41, both sidewalls 38 and 39 of the dielectric support member are exposed to an ionbeam, which due to anistropy of the geometry, mills away only thematerial from the side walls 38, 39 and leaves the thick strips at thetop and the bottom 35 and 36 substantially untouched. This results in astructure with a cross-section shown schematically in FIG. 4E.

[0111] After removal of the thin layer 42, 43 from the sides 38 and 39,thick layers 40 and 41 of Cr, Ni, W or Ta of the thickness approximately50-100 Å are left on the sides 35 and 36 of the dielectric supportmember.

[0112] Further in the following step shown in FIG. 4F, up to 10 μm thickconductive material which may include any metallic material orsuperconductor, Cr, W, Pt, Nb, Au, Ag, Cu, Al, YBCO, or mixturesthereof, is deposited on the thick layer of Cr, Ni, W or Ta 40 using thedeposition/evaporation technique shown in FIG. 4D and described inprevious paragraphs. The deposition/evaporation technique provides for adirectional deposit of conductive material onto the thick layer of Cr,Ni, W or Ta 40 which facilitates adhesion of the conductive material tothe surface of the dielectric support member.

[0113] After the layer 44 is deposited on the Cr, Ni, W or Ta underlayer40, the structure is rotated in accordance with the number of conductorsdesired, and the deposition/evaporation technique is administered to thedielectric support member to deposit a conductive layer 45 on the top ofthe Cr, Ni, W or Ta underlayer 41. It is clear that a thin layer ofconductive material can be deposited between the layers 44 and 45 whichundesirably electrically couples the layers 44 and 45 of conductivematerial. In order to remove unwanted thin layers 46 and 47 ofconductive material and to further electrically separate the layers ofconductive material 44 and 45 each from the other, both thin layers 46and 47 are exposed to an ion beam or focused ion beam which mills awaythe material from the positions between the layers of conductivematerial 44 and 45. Additionally, unwanted material may also be removedby chemical etching.

[0114] In this manner, the multi-strips structure may be created on thesurface of a dielectric support member, which may include as manyconductive strips as desired in accordance with the design of the probe.Although FIGS. 4A-4G illustrate a two conductor transmission line on thesurface of the dielectric support member, it is clear that theprinciples of the present invention are equally applied to any number ofconductors in the transmission line.

[0115] Two methods of removal of material from sides of the dielectricsupport member are envisioned in the scope of the present invention:

[0116] A. Ion beam milling which removes material from the side of thefiber wire which is facing the ion beam. Etch rates vary for differentmaterials and milling apparatus, thus in order to mill the desiredvalley areas of the dielectric support member coating, the setting andduration of milling are controlled in accordance with the particularoperation being performed.

[0117] A probe has been built, using the method described above, on a125 μm fiber wire on a 1 mm diameter quartz rod, and on a 1×1.2 mmquartz bar. In each case, a 1 μm thick gold coating was deposited on thesupport member as described and then the gold was milled from thevalleys with a 3 cm Commonwealth Scientific IBS 250 ion beam mill systemfor a duration of approximately 20 minutes on each side.

[0118] The etch rate was varied by changing the ion beam settings. Theduration and etch rate determine the amount of material removed from thedielectric support member.

[0119] A smaller probe has been built, using the method of the presentinvention on a 125 μm tapered fiber wire, on a 1 μm tapered quartz rod,and on a 1×1.2 mm tapered quartz bar, all with a 12 μm diameter at theprobing end of the fiber. A 1 μm thick gold coating on each wire wasformed and the milling was performed of the gold from the valleys with a3 cm Commonwealth Scientific IBS 250 ion beam mill system for a durationof approximately 45 minutes on each valley side. The result was atwo-conductor transmission line with a separation of less than 12 μm anda volume resolution on the order of the transmission line separation.

[0120] B. As an alternative to ion beam milling, Mask-lessphotolithography has been developed by the Applicant to remove theconductive material and Cr. Ni, W or Ta underlayer (optionally) from thevalleys. First, as shown in FIGS. 5A and 5B, a photo-resist 50 isapplied onto the entire fiber wire coated with Cr, Ni, W or Ta. Lightthen is fed through the fiber wire, as shown in FIG. 5C, resulting inthe exposure of the photo-resist 50 on the thin semitransparent sides 42and 43 of the fiber wire. The photo-resist 50 covering the thickportions 40 and 41 of the Cr, Ni, W or Ta underlayer is not affected dueto the bulk of the Cr, Ni, W or Ta at these locations blocking thelight. The fiber is then dipped in an etching solution 51 that removesthe material from the exposed thin wall sides of the conductivecoating,-as shown in FIG. 5D.

[0121] The method shown in FIGS. 5A-5D with respect to removal of Cr,Ni, W or Ta underlayer is equally applicable to removal of theconductive material from the valleys 46 and 47 shown in FIG. 4F.

[0122] Although this invention has been described in connection withspecific forms and embodiments thereof, it will be appreciated thatvarious modifications other than those discussed above may be resortedto without departing from the spirit or scope of the invention. Forexample, equivalent elements may be substituted for those specificallyshown and described, certain features may be used independently of otherfeatures, and in certain cases, particular locations of elements may bereversed or interposed, all without departing from the spirit or scopeof the invention as defined in the appended claims.

What is claimed is:
 1. A dielectric support-based probe for complexpermittivity measurements, comprising: a dielectric support member, anda multi-conductor transmission line comprising a plurality ofelectrically isolated conductive elements extending along the length ofsaid dielectric support member.
 2. The dielectric support-based probe ofclaim 1, further comprising a tapered tip portion formed at one end ofsaid dielectric support member.
 3. The dielectric support-based probe ofclaim 1, wherein said dielectric support member is a fiber optic memberhaving a central optical fiber and a cladding layer surrounding saidcentral optical fiber, said conductive elements including conductivestrips extending on said cladding layer of said fiber optic member. 4.The dielectric support-based probe of claim 3, wherein said probe isfurther used for near-field scanning optical microscopy (NSOM)measurements.
 5. The dielectric support-based probe of claim 1, whereinconductive elements include conductive strips, and wherein saidmulti-conductor transmission line includes a plurality of saidconductive strips equidistantly spaced around said dielectric supportmember.
 6. The dielectric support-based probe of claim 5, wherein saidmulti-conductor transmission line includes a pair of said conductivestrips separated 180° from the other.
 7. The dielectric support-basedprobe of claim 2, wherein the diameter of said tapered tip portion is inthe range of 50 nm to 10 μm.
 8. The dielectric support-based probe ofclaim 1, wherein the diameter of said dielectric support member is inthe range of 10 μm to 10 mm.
 9. The dielectric support-based probe ofclaim 1, wherein said conductive elements are separated one from theother by a distance of approximately 10 nm.
 10. The dielectricsupport-based probe of claim 1, wherein said conductive elements areformed of a metallic material.
 11. The dielectric support-based probe ofclaim 1, wherein said conductive elements are formed of a materialselected from the group of materials consisting of: Au, Ag, Cu, Al, Cr,W, Pt, Nb, and YBCO, and mixtures thereof.
 12. The dielectricsupport-based probe of claim 1, wherein each of said conductive elementsincludes a layer of a conductive material formed on a 50-100 Å thickunderlayer of a material selected from the group of materials consistingof Cr, Ni, W or Ta formed on said dielectric support member.
 13. Thedielectric support-based probe of claim 1, wherein said conductiveelements are formed of a superconducting material.
 14. The dielectricsupport-based probe of claim 1, wherein said dielectric support memberis made of a material selected from a group consisting of: quartz,sapphire, and glass.
 15. The dielectric support-based probe of claim 1,wherein said dielectric support member is a dielectric rod.
 16. Thedielectric support-based probe of claim 1, wherein said dielectricsupport member is a single barrel dielectric tube.
 17. The dielectricsupport-based probe of claim 1, wherein said dielectric support memberis a multi-barrel dielectric tube.
 18. The dielectric support-basedprobe of claim 16, further comprising an optical fiber inserted into atleast one channel extending internally along said dielectric tube. 19.The dielectric support-based probe of claim 16, wherein said dielectricelements include metal wires, each inserted into at least one channelformed in said dielectric tube and extending along the length thereof.20. The dielectric support-based probe of claim 15, wherein saiddielectric support member has a cross-section thereof selected from thegroup of shapes, consisting of: circle, rectangle, pentagon, hexagon,and octagon.
 21. A method for manufacturing dielectric support-basedprobes having multi-conductor transmission line for complex permittivitymeasurements, including the steps of: (a) anisotropically depositingonto a dielectric support member at predetermined locations thereof aconductive material extending along the length of said dielectricsupport member, and (b) removing said conductive material from saiddielectric support member between said predetermined locations thereof,thereby forming a plurality of electrically isolated conductive stripsextending around said dielectric support member.
 22. The method of claim21, wherein said dielectric support member is a fiber optic wire,further comprising the steps of: prior to anisotropical deposition ofthe conductive material, removing an outer jacket from said fiber opticwire of predetermined length, thereby exposing a cladding layersurrounding a central optical fiber of said fiber-optic wire.
 23. Themethod of claim 21, further comprising the steps of: forming a taperedtip portion of said dielectric support member and forming an aperture insaid tapered tip portion by removing said conductive material from saidtapered tip portion by means of a material removal technique selectedfrom the group consisting of Ion Beam Milling, Focused Ion Beam Milling,and Chemical Etching.
 24. The method of claim 23, further including thesteps of: in said step of Ion Beam Milling of said aperture in saidtapered tip portion of said dielectric support member, positioning saiddielectric support member in predetermined mutual disposition with anion beam, wherein the apex of said tapered tip portion faces said ionbeam, and anisotropically milling said tapered tip portion to create anaperture with a diameter on the order of said apex curvature.
 25. Themethod of claim 21, further comprising the steps of: prior to said step(a), anisotropically depositing a 50-100 Å thick underlayer of amaterial selected from the group of materials, consisting of Cr, Ni, Wand Ta directly onto said dielectric support member.
 26. The method ofclaim 25, wherein said conductive material and said thick underlayer aredeposited by a deposition technique selected from a group including:Pulsed Laser Deposition, Evaporation, Dipping, Sputtering, andElectroplating, and a direct write technique employing Lasers andFocused Ion Beams.
 27. The method of claim 21, wherein said conductivematerial is removed between said predetermined locations by a materialremoval technique selected from the group consisting of Ion BeamMilling, Focused Ion Beam Milling, and Chemical Etching.
 28. The methodof claim 22, wherein said conductive material is removed between saidpredetermined locations by means of Mask-Less Photo-Lithography, saidMask-Less Photo-Lithography comprising the steps of: applying aphotoresist layer onto a surface of said conductive material, applyinglight to said central optical fiber, thus causing anisotropical exposureof said photoresist layer to the light through said conductive materialdeposited, and dipping said fiber wire into a etching solution, therebyallowing etching and removal of said deposited conductive material atlocations between said predetermined locations.